Founded in February 2005, the free video-sharing Web service is the killer application of the cycle.

Consider these facts:

First, YouTube is stealing time and attention from older broadcast media. In the 60 years that the top three TV networks have been broadcasting, they have produced about 1.5 million hours of content — assuming, for argument’s sake, that they have been broadcasting 24 hours a day, 365 days a year. That’s the equivalent of about six months’ worth of YouTube uploads.

Kansas State University professor Michael Wesch points out that the amount of content currently pouring onto YouTube — about 200,000 three-minute videos added every day — is the equivalent of 385 always-on TV channels. In July 2008 in the United States, approximately 91 million viewers looked at nearly 5 billion videos on YouTube. The typical viewer watched 55 videos on the site in that month, according to Comscore.

Second, the service has become the ubiquitous resource for all things political. According to the Pew Research Center, as of this past spring, 35 percent of all Americans said they had watched online videos related to the campaign, triple the level of 2004. One out of 10 Internet users — people who told Pew that they at least occasionally use the Net or send or receive e-mail — said they have either forwarded or posted someone else’s political video.

YouTube is used both for instant response and for deep recall. Every day, the candidates and their supporters upload snippets of video from an event, sometimes even before the event concludes, in the hopes of catching the attention of the networked public sphere. At the same time, people are searching for and watching videos of content they may have missed but want to catch up on — whether it’s the latest “Saturday Night Live” parody or an old speech by a candidate. YouTube has become the instant replay or, more appropriately, the TiVo of our time.

Third, YouTube has made it possible for the presidential campaigns to become full-fledged media operations, delivering their messages directly to voters without any intermediaries such as the mainstream media — and at a fraction of the price of paid TV ads.

Together, Barack Obama and John McCain’s YouTube channels have received more than 100 million video views — though it’s telling to look at the different ways the two campaigns use the service. As of this writing, McCain’s channel contained just over 300 videos, with about 20 million views in all. Nearly all of his videos are short, well-produced pieces that look like, and in many cases are, TV commercials. Of his top 10 most-viewed videos, only one of them — a nearly eight-minute clip of a Sarah Palin speech — breaks that mold.